docs: def format_hep(citation_elements):
      """Format hep-th report numbers with a dash
  
      e.g. replaces hep-th-9711200 with hep-th/9711200
      """
      prefixes = ('astro-ph-', 'hep-th-', 'hep-ph-', 'hep-ex-', 'hep-lat-',
                  'math-ph-')
      for el in citation_elements:
          if el['type'] == 'REPORTNUMBER':
              for p in prefixes:
                  if el['report_num'].startswith(p):
                      el['report_num'] = el['report_num'][:len(p) - 1] + '/' + \
                          el['report_num'][len(p):]
      return citation_elements